* Bump maintenance branches to 3.0.2, 2.7.4, and 2.6.8 (#805)

These Rubies has been released.

- https://www.ruby-lang.org/en/news/2021/07/07/ruby-3-0-2-released/
- https://www.ruby-lang.org/en/news/2021/07/07/ruby-2-7-4-released/
- https://www.ruby-lang.org/en/news/2021/07/07/ruby-2-6-8-released/

## Ruby 3.0 branch

Bump 3.0 branch from 3.0.1 to 3.0.2

ruby/ruby@v3_0_1...v3_0_2

ruby/ruby@110f242
listed in #800 (comment)
is backported, but the issue has not yet been resolved at ruby31.y of Parser gem.

## Ruby 2.7 branch

Bump 2.7 branch from 2.7.3 to 2.7.4

ruby/ruby@v2_7_3...v2_7_4

There seems to be no change to the syntax.

## Ruby 2.6 branch

Bump 2.6 branch from 2.6.7 to 2.6.8

ruby/ruby@v2_6_7...v2_6_8

There seems to be no change to the syntax.

.github/workflows/test.yml

@@ -17,14 +17,14 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ["2.4.10", "2.5.9", "2.6.7", "2.7.3", "3.0.1", "jruby-9.2"]
+        ruby: ["2.4.10", "2.5.9", "2.6.8", "2.7.4", "3.0.2", "jruby-9.2"]
         test_command: ["bundle exec rake test"]
         include:
           - ruby: "head"
             test_command: "bundle exec rake test || true"
           - ruby: "truffleruby"
             test_command: "bundle exec rake test || true"
-          - ruby: "3.0.1"
+          - ruby: "3.0.2"
             test_command: "./ci/run_rubocop_specs || true"
     steps:
     - uses: actions/checkout@v2

lib/parser/current.rb

@@ -66,7 +66,7 @@ def warn_syntax_deviation(feature, version)
     CurrentRuby = Ruby25
 
   when /^2\.6\./
-    current_version = '2.6.7'
+    current_version = '2.6.8'
     if RUBY_VERSION != current_version
       warn_syntax_deviation 'parser/ruby26', current_version
     end
@@ -75,7 +75,7 @@ def warn_syntax_deviation(feature, version)
     CurrentRuby = Ruby26
 
   when /^2\.7\./
-    current_version = '2.7.3'
+    current_version = '2.7.4'
     if RUBY_VERSION != current_version
       warn_syntax_deviation 'parser/ruby27', current_version
     end
@@ -84,7 +84,7 @@ def warn_syntax_deviation(feature, version)
     CurrentRuby = Ruby27
 
   when /^3\.0\./
-    current_version = '3.0.1'
+    current_version = '3.0.2'
     if RUBY_VERSION != current_version
       warn_syntax_deviation 'parser/ruby30', current_version
     end